As DRAMs increase in memory cell density, there is a continuing challenge to maintain sufficiently high storage capacitance despite decreasing cell area. Additionally, there is a continuing goal to further decrease cell area. One principal way of increasing cell capacitance is through cell structure techniques. Such techniques include three-dimensional cell capacitors, such as trenched or stacked capacitors. Yet as feature size continues to become smaller and smaller, development of improved materials for cell dielectrics as well as the cell structure are important. The feature size of 256 Mb DRAMs and beyond will be on the order of 0.25 micron or less, and conventional dielectrics such as SiO2 and Si3N4 might not be suitable because of small dielectric constants.
Highly integrated memory devices, such as 256 Mbit DRAMs, are expected to require a very thin dielectric film for the 3-dimensional capacitor of cylindrically stacked or trench structures. To meet this requirement, the capacitor dielectric film thickness will be below 2.5 nm of SiO2 equivalent thickness.
Insulating inorganic metal oxide materials (such as ferroelectric materials, perovskite materials and pentoxides) are commonly referred to as “high k” materials due to their high dielectric constants, which make them attractive as dielectric materials in capacitors, for example for high density DRAMs and non-volatile memories. In the context of this document, “high k” means a material having a dielectric constant of at least 20. Such materials include tantalum pentoxide, barium strontium titanate, strontium titanate, barium titanate, lead zirconium titanate and strontium bismuth tantalate. Using such materials enables the creation of much smaller and simpler capacitor structures for a given stored charge requirement, enabling the packing density dictated by future circuit design.
One class of high k materials comprises ABO3, where “A” is selected from the group consisting of Group IIA and Group IVB metal elements and mixtures thereof, and where “B” is selected from the group consisting of Group IVA elements and mixtures thereof. Such materials can be deposited by chemical or physical vapor deposition methods.
Certain high k dielectric materials have better current leakage characteristics in capacitors than other high k dielectric materials. In some materials, aspects of a high k material which might be modified or tailored to achieve a highest capacitor dielectric constant possible will unfortunately also tend to hurt the leakage characteristics (i.e., increase current leakage). For example, one class of high k capacitor dielectric materials includes metal oxides having multiple different metals bonded with oxygen, such as the barium strontium titanate, lead zirconium titanate, and strontium bismuth titanate referred to above. For example with respect to barium strontium titanate, it is found that increasing titanium concentration as compared to barium and/or strontium results in improved leakage characteristics, but decreases the dielectric constant. Accordingly, capacitance can be increased by increasing the concentration of barium and/or strontium, but unfortunately at the expense of increasing leakage. Further, absence of titanium in the oxide lattice creates a metal vacancy in such multimetal titanates which can increase the dielectric constant, but unfortunately also increases the current leakage.